I just finished a Ex 2000 to 2003 upgrade, and everything went great. I have heard that the move from 5.5 to 2000 is a sticky one, so I would guess that you should expect some troubles moving from 5.5 to 2003. I would purchase a book to help me through it. One good thing, I have read that if this upgrade is done correctly you will not lose much, if anything.
BTW, I also upgraded Win 2000 Server to Win 2003 Server. Very easy upgrade with no hiccups. Microsoft has done a good job with both Ex 2003 and Windows Server 2003.
Might want to free up your schedule for a couple of days anyways! You know how upgrades always seem to throw surprises at you.
Matt